The present invention relates to a high-frequency (HF) generator with an adjustable means of limiting the effective output power, intended in particular for HF surgical procedures including the cutting and coagulation of human or animal tissue, and also relates to a method of limiting the output power of the high-frequency generator.
In high-frequency surgery, the HF generators used for cutting and coagulating human or animal tissue are distinguished by the fact that they provide a HF voltage which, when applied to an electrode, produces an electrical arc between electrode and tissue that has the effect of making an incision in the tissue. The HF voltages between electrode and tissue that are required for this purpose have a minimal value of ca. 200 Vp (volt peak). During such cutting and coagulation, the voltage applied between the electrode and the tissue has a decisive influence on the amount of coagulation at the edges of the incision. To keep the coagulation degree constant, HF generators are provided with a control circuit that regulates the HF output voltage from the HF generator, and hence the intensity of the electrical arc between electrode and tissue, so as to maintain a constant voltage or intensity level.
The effective output power, which depends on the output voltage of the HF generator, is given by the equation
  P  =            U      2        R  and hence is also a function of the load impedance R. If the load impedance is reduced, owing for instance to a large incision area, the result is that the output voltage U can be kept constant only for as long as the HF generator can generate the necessary effective output power. As soon as the HF generator reaches its power limit, it is no longer capable of maintaining the desired constancy of the output voltage U or the intensity of the electrical arc. This is the case in particular when the effective output power (or output current) of the HF generator reaches a maximum, i.e. an upper limit that has been specified by a prior adjustment process.
For the cutting and coagulation effect, the peak value of the output voltage is particularly crucial. As is well known, the peak value of the HF voltage must reach at least 200 V, so as to ignite the electrical arc required in order to achieve the cutting effect.
In known HF generators the HF output voltage is automatically reduced when the prespecified maximal output power is reached or exceeded, so that the effective output power of the generator does not increase beyond the generator's power limit. At the same time, however, an effort should be made to keep constant the peak value of the output voltage, and hence the intensity of the electrical arc, so that the output voltage will have a largely constant influence on the degree of scab production at the edges of the incision.